1. Field of the Invention
The present invention relates to an image-forming apparatus of an electrophotographic system, a hybrid developing device for use in the image-forming apparatus, and a hybrid developing carrier to be used in the developing device.
2. Description of the Related Art
With respect to the developing system used for an image-forming apparatus of the electrophotographic system, a mono-component developing system that uses only a toner as a main component of a developer and a two-component developing system that uses a toner and a carrier as main components of a developer have been known.
A developing device of the mono-component developing system is provided with a toner supporting member that supports and transports toner and a frictional charging member that is made in contact with the toner supporting face of the toner supporting member. Upon passing through the contact position to the frictional charging member, the toner, supported on the toner supporting member, is made in friction-contact with the frictional charging member to be formed into a thin layer, and also charged to a predetermined polarity. In this manner, since the charging process of the toner is carried out by using the frictional contact to the frictional charging member, the mono-component developing device has advantages in that its structure has a small size, and is simple and inexpensive. However, since the toner is subjected to a strong stress at the contact position to the frictional charging member, the toner tends to deteriorate, and the toner chargeability is consequently degraded comparatively earlier. Due to a contact pressure between the toner supporting member and the frictional charging member, the toner tends to adhere to the two members to cause degradation in the toner charging ability, resulting in a comparatively short service life of the developing device.
In a developing device of the two-component developing system, since, by making the toner and carrier in frictional contact with each other, the two components are charged to predetermined polarities, the stress to be applied to the toner is comparatively small in comparison with that in the mono-component developing device. Since the surface area of the carrier is comparatively larger than that of the toner, the device is less subject to toner adhesion and the resulting contamination. However, since the carrier moving speed is higher than that of a latent image-supporting member in the developing area, a phenomenon in which the toner developed on the latent image-supporting member is scraped by the carrier, that is, a so-called scavenging, tends to occur to cause an image loss.
Japanese Patent Application Laid-Open No. 2006-308687 has proposed one example of a hybrid developing system in which, as shown in FIG. 11, from a developer that contains a toner 202 and a carrier 203 held on the outer circumferential face of a magnetic roller 201, only the toner is selectively supplied to the outer circumferential face of a developing roller 204, and by using the toner held on the outer circumferential face of this developing roller, an electrostatic latent image (electrostatic latent image portion) on a photosensitive member 205 is developed. A magnetic material particle, such as ferrite, a coat-type carrier formed by simply placing a resin coat film on the surface of each magnetic material particle, and a dispersion-type carrier formed by dispersing magnetic powder in a binder resin are used as carriers. In this hybrid developing system, the stress, exerted in the mono-component developing system upon charging the toner, can be reduced, and the image loss due to the carrier upon developing in the two-component developing system can be prevented.
In the hybrid developing system, in a toner supply/recovery area between the magnetic roller 201 and the developing roller 204, the carrier has to recover residual toner on the developing roller 204, while supplying the toner. However, a problem arises in which the recovering process of the residual toner by the carrier is not carried out sufficiently. When the recovery of the residual toner is insufficient, with the amount of toner on the developing roller being irregular, the toner is supplied thereon, so that a problem of so-called ghost in which the irregularity appears on an image upon developing tends to occur. This is a phenomenon in which the same image as an image previously formed by one rotation of the developing roller faintly appears on a printed image.
The objective of the present invention is to provide a hybrid developing carrier that can sufficiently recover residual toner on the developing roller in the hybrid developing system, and prevent a ghost from occurring, as well as a hybrid developing device and an image-forming apparatus.